Air-velocity sensors, which are based on the so-called constant-temperature or excess-temperature method and are normally referred to as hot-film anemometers, are conventional for measuring air-flow velocities. Such air-velocity sensors are marketed by the Applicant hereof under, for instance, model designations EE60, EE61, EE62, EE65 or EE70. Such air-velocity sensors include, inter alia, two separate sensors, which are positioned in the flowing medium, e.g., air, whose flow velocity should be determined. In this connection, a first sensor, the temperature sensor, is used for determining the temperature of the flowing medium and tracks its temperature as accurately as possible. The second sensor, the heat sensor, constitutes, in principle, a temperature sensor as well and is adjusted to a constant temperature difference with respect to the first sensor by supplying electric power. The electrical heating power necessary for this represents a direct measure of the mass flow rate or the flow velocity to be determined.
Air-velocity sensors constructed in such a manner require a relatively high heating power. Thus, if such an air-velocity sensor is intended to be operated by a battery, then the result is only very short operating times. When two batteries having each an operating voltage of 1.5 V and a capacity of approximately 2600 mAh are used, the result is typically a possible operating time between just 10 and 20 hours.
An operation, which saves as much power as possible, is, for example, possible, when such an air-velocity sensor is cyclically heated and repeated cools down in the air flow in question. The current air velocity may then be deduced from the required heating time. Reference is made, for example, to U.S. Pat. No. 4,501,145, German Published Patent Application No. 36 37 497 or German Published Patent Application No. 199 39 942 for such variants of measuring air velocity.
It is an aspect of the present invention to provide a further improved air-velocity sensor, as well as an improved method for operating an air-velocity sensor in a power-saving manner.